The Global System for Mobile Communication (GSM) cellular wireless network was initially designed to support circuit-switched services, such as voice telephony. Enhanced Data for Global Evolution (EDGE) is a superset of the General Packet Radio Service (GPRS), and provides the capability for packet-based user data interchange over GSM. GPRS, EDGE and other wireless data communication protocols are commonly used for the transmission of data through business and scientific software applications such as electronic mail (e-mail), calendar updates, file transfers and Internet browsing.
The GSM specification defines an algorithm for re-selecting radio system cells. According to this method, a mobile station in a Public Land Mobile Network (PLMN) measures the signal strength of the serving cell and the neighbouring cells in the PLMN, and then selects the cell having the largest Received Signal Strength Indication (RSSI). This approach to cell re-selection is sufficient for circuit-switched voice service because GSM includes an extremely agile handover algorithm that can rapidly move a conversation amongst available radio channels should the signal quality degrade or bit error probability on one channel reach a point where it is difficult to continue communication.
This approach is inefficient for packet-based GPRS/EDGE communications. GPRS/EDGE does not possess an agile handover mechanism for switching communications amongst available cells. As a result, should signal quality on the serving cell degrade during packet-based transmission such that a TDMA multiframe is not received, it is necessary to re-initiate transmission of the lost multiframe after cell re-selection. Also, GPRS/EDGE data communications usually involve transmission along multiple adjacent TDMA timeslots. However, as the probability of finding suitable multiple adjacent cells is equal to the probability of finding a single suitable cell having multiple adjacent timeslots, it is difficult to assign multiple timeslots on adjacent cells when radio link conditions on the serving cell become degraded. As a result, GPRS/EDGE data transfers are usually started and completed on a single cell, in contrast to a circuit-switched call in which it is common to change cells several times over the course of a single minute of conversation.
Attempts have been made to provide a more efficient cell selection mechanism in GSM networks. For instance, since the length of system information sent by different cells can vary significantly, a mobile station can incorrectly interpret a long break in user data transmission time, due to lengthy system information, as poor quality of service. Lundell (U.S. Pat. No. 7,058,406) describes a method for performing cell reselection in a GSM/GPRS network in which the mobile station uses length information received from a cell to estimate the period of time required to receive system information over the control channel. The mobile station then uses the time estimate as a parameter to the cell reselection algorithm.
Yeo (US 2006/0084443) describes a method for cell selection and reselection in a GSM network, in which the mobile station receives from the network a list of available cells, and then assigns a lower priority to cells that it had previously unsuccessfully selected. The mobile station also removes from the list cells whose access was previously found to have been forbidden. The mobile station then selects an appropriate cell using conventional radio-based cell selection and reselection methods, such as signal strength measurements, on the cells remaining in the list.
Choi (US 2006/0234757) describes a method for cell selection and reselection in a GSM/GPRS network, in which the mobile station determines its current speed and location, in an idle state, using GPS location information, and then calculates a predicted location from its current speed and location. The mobile station then uses the predicted location to select a best cell from amongst adjacent cells that have similar signal strength measurements.